The disclosure relates generally to composite materials and sensors. More particularly, the disclosure relates to methods and systems directed to sensing the presence of one or more analytes of interest (e.g., volatile organic compounds) in a target fluid (e.g., the human breath).
Timely detection of health conditions can provide potentially life-saving information to patients. Human breath has been shown to contain VOC biomarkers that can be used to identify numerous diseases. These VOCs allow for many applications of noninvasive detection of diseases and other biological metrics. Acetone levels in breath are a significant indicator of the presence of diabetes, a disease that threatens the lives of millions of people in the US and around the world. Moreover, links have been reported between breath isoprene level and oxidative stress, blood cholesterol level, and increased breath and heart rate. Ethanol has been shown to have potential medical applications for diabetes detection. Esters similar to 2-ethylhexyl acetate are common in breath and have been linked to multiple diseases, including lung cancer. These and other VOCs can be used to non-invasively detect and monitor human health conditions. For example, diabetes patients experience rapid changes in their blood glucose levels throughout the day and may experience potentially life threatening conditions. However, reliable non-invasive methods for monitoring blood glucose levels currently do not exist. A sensor array that can monitor symptoms of diabetes by analyzing breath VOCs, most importantly: acetone, can offer lifesaving information for such patients.
Diabetes threatens the lives of millions of people in the US and around the world. Diabetes patients experience rapid changes in their blood glucose levels throughout the day and may experience potentially life threatening conditions. The finger prick glucometer is the most commonly employed method for glucose measurement, however, continuous glucose monitors (CGMs) have also been recently used when needed. As these and other currently utilized blood glucose measurement techniques are invasive, a non-invasive method for accurately detecting elevated blood glucose levels could provide lifesaving information to diabetic patients. Blood glucose level variation can manifest itself in many different ways in the human body. The design and fabrication of a noninvasive glucose sensor which estimates blood glucose sugar based on the analysis of volatile organic compounds (VOCs) including acetone in exhaled breath can offer lifesaving information for such patients. The interaction of red blood cells with alveolar air in the lungs leaves a foot print of the VOCs from blood in the exhaled breath. An increase in blood glucose levels changes the metabolic behavior of the body and changes the VOC footprint. Recent studies have resulted in more evidence for the existence of a linkage between exhaled acetone and blood glucose levels in diabetes patients. The findings have been supported by the fact that due to lack of insulin, ketoacidosis increases the plasma acetone concentration.
Two of the major challenges in developing breath VOC sensors are as follows: (1) a need to be highly sensitive because the concentrations of VOC biomarkers in exhaled breath are extremely low and (2) a need to be highly selective in detecting the target VOCs due to the presence of hundreds of gaseous components in breath, including water vapor. The human sense of smell works well in the presence of water as the olfactory receptors are not sensitive to water. However, sensors developed to detect VOCs often exhibit a high response to water, which interferes with the detection of the target agents. This has been a major challenge in developing electronic noses for breath VOC detection applications. These requirements have encouraged researchers to consider a wide range of materials including: CNT, graphene, carbon black, gold nanoparticles, metal oxides, and various polymers. Gold nanoparticle-based sensors have also shown promise in the detection of low-concentration breath VOCs. However, high sensitivity to water is problematic with these sensors. Conducting polymers have been shown to detect acetone at room temperature, yet these sensors often suffer from high sensitivity to moisture and performance degradation over time. Carbon-based materials, including CNT, carbon black, and graphene have also been investigated intensely for breath VOC sensing applications. Pristine CNTs do not efficiently adsorb VOCs, but functionalization improves their performance for sensor applications. Functionalization of CNTs with carboxylic groups and defect sites on CNTs created through acid sonication have shown improved adsorption of organic compounds and detection of low concentration VOCs. Interestingly, these processes also increase the response to water. Polymer-coated CNTs and polymer carbon black composites have also been utilized in detecting breath VOCs. CNT, carbon black and polymer composites have been used to achieve high sensitivity and response time. In CNT polymer composites, the swelling or contraction of polymers changes the separation between the conducting CNT material, which alters the resistance. These CNT polymer composite-based sensors have been shown to have both high sensitivity and selectivity. However, these sensors suffer from poor selectivity due to their high response to water molecules. For example, CNT/poly(ethyleneterephthalate) (PET) sensor has been reported to exhibit a 2.7 times higher responsiveness to water than to acetone. Carbon black (C65) composites with low-density polyethylene (LDPE) and poly(ethylene-block-ethylene oxide) (PE-b-PEO), polyethylene glycol (PEG), and poly methyl methacrylate (PMMA) have been used to detect acetone, however, their acetone selectivity in the presence water is still poor. Adsorption of water molecules have also been known to accelerate sensor degradation. Composites of PVDF-HFP, previously reported in battery and piezoelectric device applications, exhibit better material stability and more hydrophobic properties compared to PVDF, PMMA, or PEG. PVDF-HFP is also known to swell when exposed to acetone. However, PVDF-HFP itself is a poor electrical conductor, therefore requires infusion of conducting particles to form chemiresistive sensors.
Metal oxides have offered promising features for detecting very low concentrations of acetone. Generally, high sensitivity metal oxide based sensors are operated at an elevated temperature, resulting in higher energy consumption. Conducting composite polymers present another method for detecting acetone. The swelling effect of PMMA by acetone has been widely studied. The conductivity of PMMA can be improved by adding PPy to form a resistive based acetone sensor. A film composed of carbon nanotubes with PMMA has been reported to detect low concentrations of acetone. However, these sensors are also known to suffer from poor selectivity.
Consequently, considering such limitations of previous technological approaches, it would be desirable to have a system and method for a highly sensitive acetone sensor applicable in non-invasive detection of blood glucose level from human breath.